Power converters are used to convert an unregulated power source to a regulated voltage and/or current source. FIG. 1 shows a traditional power converter with synchronous rectification. A switching signal S1 is utilized to control the duty cycle of a switch 10 for the regulation of the output voltage VO. The output voltage VO is provided to a load 50. A charge current will charge an output capacitor 40 during the on time of the switch 10. FIG. 2 shows a switching signal S2 to enable a switch 20 in response to the off-state of the switch 10 to provide a low impedance current path for a discharge current IF of the inductor 30. A switching signal VW is applied to charge the inductor 30 once the switch 10 is turned on.
In continuous current mode (CCM) operation, the switch 10 is turned on before the energy of the inductor 30 is completely discharged. For the discontinuous current mode (DCM), the energy in the inductor 30 is fully discharged before the start of the next switching cycle. FIG. 3 shows a reverse current IR discharge the output capacitor 40 through the switch 20 during the DCM operation. The reverse current IR will cause the power loss and lower the efficiency of the power converter at light load and no load conditions. FIGS. 4A and 4B show CCM and DCM waveforms respectively, wherein the IIN is the charge current.
FIG. 5 shows a traditional forward power converter including synchronous rectifier, in which to provide the output voltage VO to a load 55. A secondary winding of a transformer 60 generates a switching voltage to turn on a rectifier 16 and charge an inductor 35. A capacitor 45 is coupled to the inductor 35. During the off period, the switching voltage is reversed, the rectifier 16 is turned off and a rectifier 26 is switched on to discharge the energy of the inductor 35. Switches 15 and 25 serve as synchronous rectifiers used to reduce the power loss of the rectifiers 16 and 26. Switching signals S3 and S4 are synchronized with the switching voltage for switching the switches 15 and 25 respectively. A switching signal VW is applied to charge the inductor 35 once the rectifier 16 is turned on. The charge current of the inductor 35 is proportional to the voltage and the pulse width of the switching signal VW. According to the voltage and the pulse width of the switching signal VW and the output voltage VO, the discharge time of the inductor 35 can be predicted to avoid the reverse current for synchronous rectification.
Prior art methods of limiting the reverse current in a synchronous rectifier includes the use of a current sensing circuit to turn off the synchronous rectifier once the reverse current is detected. The current sensing circuit involves using the turn-on resistor (RDS-ON) of the transistor (synchronous rectifier) or a series resistor to detect the reverse current. However, these current sensing circuit cause power loss and add complexity to the system. Moreover, the synchronous rectifier can only be turned off after the reverse current is generated and detected. Accordingly, a control circuit that eliminates the effects of reverse current without the current sensing circuit would be advantageous.